The resources below regard the change in the law December 20, 2019 from the passing of the Secure Act. These particulars do not fully apply to you because you took a 100% distribution. The commentary, however, seems to imply that the answer to your question would be that it was a first year withdrawal, as for you, even though your father had been taking the RMDs for some time.
The amount that is taxable depends on the basis of assets contributed by your father.
Additional Resources:
2019 New Jersey-1040 Instructions
2019 IRS Publication 590-B beginning on page 5 for Inherited IRAs
IRS Retirement Topics - Beneficiary
IRS Retirement Topics - Required Minimum Distributions (RMDs)
Inherited or "Stretch" Individual Retirement Accounts (IRAs) and the SECURE Act
